This type of electric cable needs to satisfy numerous criteria for use in aviation, in particular when placed in fire conditions.
For example, one safety criterion is to ensure that the electric cable continues to operate at high temperatures of the order of 1100° C. for some specified minimum length of time, generally of the order of 5 minutes (min) to 15 min, without its electrical conductor melting, and without propagating the fire, and that it should also withstand vibration and being sprayed with water or fire-extinguishing fluids, while continuing to ensure electrical continuity for its circuits and while conserving some minimum level of insulation resistance in flame, generally of the order of 10,000 ohms (Ω).
Other criteria can also be taken into account such as the weight and the diameter of said cable, which weight and diameter must not be excessive, the maximum temperature at which said cable can be used on a continuous basis, which maximum temperature needs to be as high as possible, generally about 260° C. for at least 20,000 hours, and the ability of said cable to be marked so as to enable it to be identified.
A more recent criterion requires a safety electric cable to operate well when associated with other electric cables to constitute a harness.
Document FR 2 573 910 describes an electric cable for aviation that comprises an electrical conductor surrounded by a first layer constituted by two windings of mica tape.
The first layer is covered in a second layer of thermostable polymer that may be constituted for example either by a polytetrafluoroethylene (PTFE) tape, or by a polyimide resin.
Finally, the second layer is covered in an intermediate layer of glass fibers, and in an outer layer of the same kind as the second layer.
Nevertheless, although that prior art electric cable satisfies the safety criteria specified above, it is not good at satisfying another safety criterion, namely that of resistance to electric arc propagation as specified by the standard NF EN 3475-604 (a method of evaluating resistance to electric arc propagation when dry) and standard EN 2346-005 (a standard defining the minimum performance required of an aviation electric cable in terms of resistance to fire and to electric arc propagation).
This safety criterion makes it possible to guarantee that the insulation of said cable presents sufficient resistance to avoid triggering and propagating electric arcs between electric cables and/or between electric cables and a conductive structure.
The technical problem to be solved by the subject matter of the present invention is to propose an electric cable that makes it possible to avoid the problems of the prior art, in particular by providing resistance to electric arc propagation that satisfies the requirements of standard EN 2346-005 concerning arc propagation testing and standard NF EN 3475-604, while maintaining the good properties of withstanding fire and operating in flame as specified in the standards NF EN 3475-408 and prEN 3475-417.
According to the present invention, the solution to the technical problem posed lies in that the electric cable comprises:                an electrical conductor surrounded by a first layer comprising at least one winding of mica tape, said mica tape being made up of mica particles deposited by means of a polymer binder on a backing;        a second layer comprising at least one winding of a polyimide tape; and        a third layer comprising at least one winding of a polytetrafluoroethylene (PTFE) tape;        
the first layer being subjected to heat treatment at a temperature of at least 400° C.; and
the ratio R of the weight per unit length of PTFE over the sum of the weights per unit length of the polymer binder and of the polyimide being such that:                R is greater than or equal to 2 when the section of the electrical conductor is no greater than 0.2 square millimeters (mm2), and preferably lies in the range 0.1 mm2 to 0.2 mm2;        R is greater than or equal to 4 when the section of the electrical conductor is strictly greater than 0.2 mm2 and strictly less than 0.6 mm2;        R is greater than or equal to 6 when the section of the electrical conductor is equal to 0.6 mm2; and        R is greater than or equal to 12 when the section of the electrical conductor is strictly greater than 0.6 mm2, and preferably no more than 3 mm2.        
The Applicant has discovered, surprisingly, that for a given range of electrical conductor sections, imparting specific heat treatment to the first layer in combination with a ratio R of weight per unit length of PTFE over the sum of the weights per unit length of the polymer binder plus the polyimide makes it possible to achieve dry electric arc propagation resistance of more than 75%, as specified by the standards NF EN 3475-604 and EN 2346-005.
In addition, the electric cable most advantageously retains very good resistance to fire and ensures electrical circuit continuity well, while presenting weight and diameter that are relatively small, so as to satisfy the criteria required in aviation.